1. Field of the Invention
The present invention is directed to a process for purifying aqueous alkali metal chlorate solutions containing alkaline earth metal-containing impurities. In particular, the present invention is directed to processes for separating and removing alkali earth metal impurities from an aqueous alkali metal (e.g. sodium) chlorate solution using selected precipitation and ion exchange techniques to effect such removal and separation. Furthermore, the present invention is directed to a process for regenerating with chloric acid the ion exchange material used in effecting the alkaline earth metal impurity separation and removal from alkali metal chlorate solutions.
2. Description of Related Art
The purity of the aqueous solution of an alkali metal chlorate feedstock to an electrolytic cell used for producing chloric acid or aqueous mixtures of chloric acid and alkali metal chlorate is an important factor in the long term operation of the electrolytic process. It has also been found that a sodium chlorate feedstock solution which contains substantial amounts of total magnesium and calcium impurities of about 0.5 parts per million (ppm) by wt. or greater may also cause an undesirable voltage rise during the electrolytic cell operation over time. Accordingly, it is desirable to remove these alkaline earth metal impurities from the alkali metal chlorate feedstock solution before the electrolytic cell operation. The present invention provides an answer to this need.
Separately, there are numerous references concerned with the removal of calcium and magnesium impurities from sodium chloride brine. Illustrative of such references are included in the following patents and publications:
U.S. Pat. No. 4,072,472, which issued to Lukes on February, 1978, teaches a process for removal of calcium and sulfate impurities from NaCl subterranean deposits involving insolubilizing the calcium values.
U.S. Pat. No. 4,196,140, which issued to Lynch on Apr. 1, 1980, is directed to a process for recovering chlorine from an aqueous waste stream containing an alkali earth metal halide, alkali earth metal hypohalite, and an alkaline earth metal chlorate. This process involves the following steps:
(1) separating the unpure solution with an alkali metal hydroxide to form a slurry of solid particles of alkaline earth metal hydroxide suspended in a liquid; PA1 (2) separating the alkali earth metal particles-from the liquid; and PA1 (3) admixing the liquid with an organic alcohol. PA1 (a) contacting the brine with solid particles of magnesium-containing silicate to cause said cationic contaminants to adhere to the magnesium-silicate particles and PA1 (b) then separating the resulting solid particles of magnesium-containing silicate having said cationic contaminants adhering thereto from the resulting purified brine. PA1 (1) admixing an alkali metal bicarbonate with the alkaline brine to form an insoluble calcium salt; PA1 (2) removing the insoluble calcium salt from the brine; and PA1 (3) recovering a purified alkali metal chlorine brine having reduced calcium ion concentration. The purified alkali metal chloride brine having a calcium ion concentration from about 0.5 to about 2.0 parts per million by weight. PA1 (a) adding an alkali metal oxalate compound to said brine to form an insoluble calcium oxalate compound; PA1 (b) separating said insoluble oxalate precipitate from said brine to purify said brine; PA1 (c) recovering the resulting purified brine; and PA1 (d) contacting said purifying brine with an ion exchange resin to further reduce the content of calcium ions in said purified brine. PA1 (a) collecting an impure alkali metal halide solution containing halates and hypohalites therein; and PA1 (b) contacting the collected solution with at least a stoichiometric amount of an acid and an aldehyde to reduce substantially all of the alkali metal hypohalite and halate within said solution to halogen and alkali metal halide. PA1 (a) adjusting the pH of said purified brine to between about 2.0 and about 3.0; PA1 (b) contacting said brine with a strong macroreticular cationic chelating resin to remove said dissolved aluminum and silica therefrom; and PA1 (c) separating the purified brine from the resin. PA1 (a) treating a portion of resaturated brine liquor at a temperature of about 90.degree. C. to about 105.degree. C. with at least a stoichiometric amount of hydrochloric acid to reduce essentially all of the alkali metal halate within said portion to halogen and alkali metal halide. PA1 (a) adjusting the pH of the brine to a level of between 4 and 12 (preferably by adding an alkali metal hydroxide); PA1 (b) operating an electrolytic membrane cell under certain conditions to maintain the pH of said brine during electrolysis at a value above 3.5. PA1 (a) adjusting the pH of the alkali metal halide-brine to 8.5-9.5; and PA1 (b) contacting the pH adjusted alkali metal halide brine with an ion exchange resin having a phosphonic acid group. PA1 (a) electrolyzing a sodium chloride brine stream containing calcium impurities in an electrolysis cell to produce sodium chlorate solution; PA1 (b) passing said sodium chlorate into a crystallizer to produce sodium chlorate crystals and a mother liquid containing calcium impurities; PA1 (c) separating the crystals from the mother liquor; PA1 (d) passing at least a portion of said mother liquid through a cationic chelating ion exchange resin column in sodium form; PA1 (e) operating said column so that, on the average, it removes only an amount of calcium equal to the amount of calcium in the brine steam thereby preventing the buildup of calcium therein; PA1 (f) recycling the calcium-depleted mother liquor through the electrolysis cell. PA1 (1) adding a sufficient amount of alkali compound to said chlorate ion-containing solution to adjust the pH of said solution to above about 9 and to cause the precipitation of alkaline earth metal compounds in said solution; said alkali compound selected from an alkali metal carbonate, alkali metal bicarbonate, alkali metal hydroxide, and mixtures thereof; and PA1 (2) separating said pH adjusted aqueous alkali chlorate solution from said alkaline earth metal precipitates, thereby forming a first purified alkali metal chlorate solution having less than about 0.3 parts per million by weight of total alkaline earth metal ions. PA1 (3) contacting said first purified alkali metal chlorate solution with a chelating ion exchange resin to remove further amounts of alkaline earth metal impurities, thereby producing a second purified alkali metal chlorate solution having less than about 0.05 parts per million by weight of total alkaline earth metal ions. PA1 (4) regenerating said chelating ion exchange resin with chloric acid to remove said alkaline earth metal impurities from said chelating ion exchange resin. PA1 (1) contacting a chelating ion exchange resin contaminated with alkaline earth metal impurities with an aqueous chloric acid solution and removing said impurities from said resin; and PA1 (2) then contacting said resin with an aqueous solution selected from the group consisting of (a) water, (b) an alkali metal hydroxide solution, (c) an alkali metal chlorate solution, and (d) mixtures thereof, thereby removing substantially all of the remaining chloric acid from said chelating resin. PA1 (1) contacting said impure aqueous chlorate ion-containing solution with a chelating ion exchange resin, thereby producing a purified alkali metal chlorate solution having less than about 0.3 parts per million parts by weight of total alkaline earth metal impurities and leaving said alkaline earth metal impurities on said chelating resin; and PA1 (2) regenerating said Chelating resin with an aqueous chloric acid solution to remove said alkaline earth metal impurities from said resin. PA1 (1) passing a purified aqueous alkali metal chlorate solution having less than about 0.3 parts per million parts by weight of total alkaline earth metal impurities therein through an electrolytic cell having a cation exchange membrane to produce an aqueous solutions containing chloric acid and an aqueous solution having alkali metal hydroxide.
U.S. Pat. No. 4,207,152 which issued to Kadija et al. on Jun. 10, 1980, teaches a process for purifying an alkali metal chloride brine by removing cationic contaminants comprising:
U.S. Pat. No. 4,277,447, which issued to Chambers et al. on Jul. 7, 1981, teaches a process for reducing calcium in concentration from alkali metal chloride brines containing an alkali metal hydroxide comprising:
U.S. Pat. No. 4,303,624, which issued to Dotson et al. on Dec. 1, 1981, is directed to a method of purifying alkali metal chloride brine containing calcium ion impurities for introduction into a electrolytic cell comprising:
U.S. Pat. No. 4,308,030, which issued to Smith et al. on Dec. 29, 1981, claims a method for determining low level concentrations of alkaline earth metals in a concentrated aqueous alkali metal chloride brine.
U.S. Pat. No. 4,397,720, which issued to Moore et al. on Aug. 9, 1983, is directed to an improved process for purifying an alkali metal halide brine liquor comprising:
U.S. Pat. No. 4,450,057, which issued to Kelly on May 22, 1984, is directed to a process for removing dissolved aluminum and silica impurities from an alkali metal halide brine comprising:
The patent also teaches regeneration of chelating by (d) washing the resin with a mineral acid to remove said dissolved aluminum and silica therefrom and (e) washing said acidified resin with a caustic solution so that it can absorb positive ions.
U.S. Pat. No. 4,481,088, which issued to Moore et al. on Nov. 6, 1984, is directed to the removal of chlorate ions from a recirculating anolyte brine comprising:
U.S. Pat. No. 4,515,665, which issued to Fair et al. on May 7, 1985, claims a process for stabilizing a complex of metal and silica in an alkali metal halide brine comprising:
U.S. Pat. No. 4,702,805, which issued to Burkell et al. on Oct. 27, 1987, is concerned with a continuous process for producing sodium chlorate by the electrolysis of sodium chloride contaminated with sodium sulfate which involves (1) cooling the resulting sodium chlorate liquor to crystallize out a portion of said sodium crystals, thereby providing purified sodium chlorate crystals and a resulting mother liquor containing the sulfate impurities; (2) separating the purified crystals from the mother liquor and (3) recycling to the electrolysis cell.
U.S. Pat. No. 4,747,917, which issued to Reynolds et al. on May 31, 1988, is directed to a process for reducing sulfate ion concentration in an alkali metal halide brine.
U.S. Pat. No. 4,830,837, which issued to Justice et al. on May 16, 1989, is directed to a process for purifying an alkali metal halide brine containing aluminum as an impurity which comprises:
U.S. Pat. No. 4,966,764, which issued to Reed et al. on Oct. 30, 1990, teaches a process for purifying a dehalogenated alkali metal halide brine containing aluminum impurities.
U.S. Pat. No. 5,023,803, which issued to Loftis et al. on Jun. 11, 1991 teaches a process which controls the amount of excess carbonate added to the alkali metal brine system to substantially remove by precipitation all of the calcium impurity in the brine as a calcium carbonate precipitated flocculent.
U.S. Pat. No. 5,082,567, which issued to Fritts et al. on Jan. 21, 1992, teaches the regeneration of a cationic exchange resin that had been used to remove chlorate ions from an aqueous solution by (1) rinsing the resin with water and (2) contacting the resin with an aqueous acidic sulfite solution.
U.S. Pat. No. 5,104,500, which issued to Ruthel on Apr. 14, 1992, teaches an improved method of producing sodium chlorate crystals comprising:
U.S. Pat. No. 5,169,406, which issued to Tewari on Dec. 8, 1992, teaches a process for preparing a sodium chloride brine solution from a sodium chloride salt containing calcium sulfate impurities in a certain apparatus.
European Patent Application, which was filed by Eka Nobel AB on Jan. 24, 1992 and was published on Aug. 12, 1992, teaches a process for electrolytic production of sodium chlorate wherein alkali metal hydroxide co-produced is used for the precipitation of impurities and regeneration of ion-exchange resins in connection with dissolution and purification of technical alkali metal chloride.
Russian Patent Document No. 994407, entitled "Method of Purifying Sodium Chloride Solutions" by K. Z. Rogozovskaya et al., teaches subjecting a sodium chloride solution with a soda-caustic treatment to precipitate out calcium carbonate and magnesium and iron hydroxides.
United Kingdom Patent Specification No. 1,519,571, which was filed by Allied Chemical Corp. on Jan. 5, 1977 and published on Aug. 2, 1978, teaches a process for purifying sodium chloride brine by reacting said brine sequentially with sodium carbonate and sodium hydroxide, removing the precipitating solids, and then recycling a portion of said removed solids to the reaction zone of the starting brine with the sodium carbonate.
H. Maeda and H. Egawa, "Studies of Selective Adsorption Resins XXVI, Removal of Calcium and Magnesium Ions in a Salt Solution with Chelating Resin Containing Aminomethylphosphonic Acid Groups", J. Appl. Polym. Sci., Volume 39, No. 7 pages 1519-25 (1990) teaches the adsorption mechanisms of calcium and magnesium ions from a sodium chloride salt solution onto a selected chelating resin.
R. J. Coin "Brine Treatment" presented at the Ninth Annual Chlorine/Chlorate Seminar on Oct. 5-7, 1993 teaches various processing parameters investigated during the purification of sodium chloride brines.
All of the above references are incorporated herein by reference in their entireties.
In practice, these prior art precipitation processes used in treating alkali metal chloride brine using alkali metal carbonate, alkali metal hydroxides, and mixtures thereof have been able to remove alkaline earth impurities to total alkaline earth metal concentrations of only about 0.5 to 1.5 parts per million by weight. Unexpectedly, it has now been found that alkali metal chlorate solutions behave differently from alkali metal chloride brine solutions such that under specific processing conditions, the amount of alkaline earth metal in these chlorate solutions can be reduced to concentrations significantly lower than 0.5 ppm and actually to concentrations less than 0.1 ppm.
The solubility of calcium and magnesium have been experimentally found to increase with increasing temperature in alkali metal chloride brine solutions. Accordingly, carrying out calcium and magnesium precipitation treatments with an alkali metal chloride brine at elevated temperatures results in more impurities in the brine solution. To the contrary, it has now been found that temperatures from about 30.degree. C.-100.degree. C. do not adversely affect the amount of Ca and Mg precipitation in an alkali metal chlorate solution and in fact help in reducing the amount of chemicals required to achieve low concentrations of calcium and magnesium. Furthermore, such higher temperatures have additional advantages since they increase the rate of reaction and thus decrease the time needed for the precipitation.